Polymeric concrete for wind generator towers or other large structural applicatons

ABSTRACT

The present invention relates to towers for wind generators or other large structural applications and uses a new construction concept, based on polymeric concrete. Polymeric concrete is composed of thermosetting resin and aggregates such as sand or gravel. Polymeric concrete has low maintenance costs and exceptional high resistance to corrosion, thus justifying its main usage in non-structural applications. Additionally, polymeric concrete has been used as mortar in the rehabilitation of civil structures, especially retrofitting of bridges and heritage buildings. Its advantages for these applications are the adherence to the traditional materials, higher compressive strength than traditional concrete and low specific weight. The tower ( 1 ), according to the invention, is built of two or more superimposed ring sections ( 2 ) in conical or cylindrical shape, each ring ( 2 ) being built of one or more shell segments and the said segments being fixed by means of mechanical and/or chemical couplings, and being made of prefabricated polymeric concrete.

TECHNICAL DOMAIN

The present invention relates to towers for wind generators or otherlarge structural applications and applies a new construction concept,based on polymeric concrete.

STATE OF THE ART

Wind generators have gained wide acceptance as an alternative source forthe production of renewable and clean energy. In recent years, state ofthe art wind energy converters had a dramatic development in energyoutput/production, by using longer blades and more powerful generators.Rotor diameter of state of the art units has reached 120 m and generatorpower has reached 5 MW. Wind generators are supported to a convenientheight by towers, in order to expose them to a convenient wind flow andprevent interaction between the rotor blades and the ground. The towersthemselves are adequately attached to the foundations. The developmenttrend described above requires increased hub height, and tower heightfor the same state of the art unit has reached 120 m.

These towers must support the increased weight of the energy converters,withstand the wind forces on the whole unit and provide adequatemechanical resistance to the dynamic behaviour of the generator,including stiffness and fatigue wear for a minimum 20 year life time.Thus they are a demanding design project representative of engineeringstate of the art for supporting large and heavy structures at extremeheights.

The same energy converters have also been applied off-shore, where windflows are more convenient and where there are fewer implications withground occupancy. The supporting towers have been attached to steelpylons or concrete foundations that reach above maximum sea water level.

The towers have a significant impact in the overall cost of the windgenerator unit and several solutions have been proposed both to supportthe development trend to higher hub heights and to reduce the costs inmanufacture, transport, assembly and maintenance, which become more andmore relevant with increasing height. A variety of construction methods,from guyed poles, steel wall, steel lattice, concrete wall, hybrid steelwall & lattice, hybrid concrete & steel wall to composite materials,have been proposed for wind towers. A viable solution has to providenecessary mechanical resistance both to static and dynamic loads atincreasing heights and prove cost-effectiveness in manufacture,transport, assembly and maintenance.

The industry has used mainly steel towers, made of cylindrical orconical sections of metal wall, flanged at the extremities, the sectionsbeing bolted together on site. However, as increased tower heightimplies bolder dimensioning of the tower diameter, of the wallthickness, or both, in order to assure the necessary stiffness, thissolution has met increasing limitations due to materials costs and alsomanufacture and transport limitations due to dimensions, as a diameterof 4.2 m is the maximum allowed in many roads due to over-crossings.This is the main reason why the standard steel tower height for themulti-MW generators today is still 80 m, which was the state of the art10 years ago.

Towers of steel lattice construction were used for smaller towers in thepast and have also been proposed for higher towers. Lattice towers needmore ground space and imply more time consumption for on site assembly.They also have increased inspection and maintenance costs. A varianthybrid construction, made of a lower section of steel lattice and anupper section of steel wall, has been proposed as in DE 103 39 438 A1,but it requires expensive joints.

Proposals have been made in order to improve the steel wall towerconstruction presently used by the industry. The construction of thetower has been proposed in variable forms, including various numbers ofring sections and shell segments, as in WO 2004/083633 A1. Otherproposals are based on a more complex wall profile, as in EP 1561883 A1,providing more stiffness but requiring additional and costly operationsboth for production and on site assembly. Hybrid variants with aconcrete lower section, as in WO 2005/015013, improve the load-bearingcapacity and easiness of attachment to the foundation, thus allowinghigher towers, but they present increased costs and fail to solve themaintenance issues.

Furthermore, all the steel wall and lattice solutions requiremaintenance of the tower during the project lifetime of the windgenerator, due to corrosion. Although corrosion usually appears late inthe 20 year life-cycle of the on-shore tower, control and maintenance ofcorrosion spots on the external wall or in lattice construction entailimportant costs and risks. In more aggressive environmental conditions,such as off-shore towers, corrosion becomes an even more importantconcern and implies increased maintenance costs and risks.

A different approach is that of concrete towers, made of pre-castconcrete sections reinforced with pre-tensioned cables. They have beenused for towers with heights of about 95 m and higher, as in DE 100 33845 A1 and DE 101 60 306 B4 and they have the advantage of a betterweathering resistance. However, this solution is considerably heavierthan the one of steel wall, with corresponding higher logistic costs andlonger on-site assembly time.

SUMMARY OF THE INVENTION

The present invention provides a solution for building large scaletowers, including towers beyond 80 m height, reducing substantially themaintenance needs, avoiding the logistics restrictions of the maximumtransportable diameter and reducing the production costs of the currentstate of the art steel tower solutions, through the application ofpolymeric concrete.

Polymeric concrete is composed of thermosetting resin and aggregatessuch as sand or gravel. Polymeric concrete has low maintenance costs andexceptional high resistance to corrosion, justifying its main usage innon-structural applications and in small size parts where corrosion isthe main problem. Complementary, polymeric concrete has beenincreasingly used in recent years in the rehabilitation of civilstructures, especially retrofitting of bridges and heritage buildingsusing polymeric mortar. Its advantages for these applications are theadherence to the traditional materials, a compressive strength higherthan traditional concrete and low specific weight.

Ongoing research has shown that an adequate amount of aggregates such asdry sands with a fine grain and gravel, with proper sizes, combined withlow viscosity resin, can provide very good adhesion and compressionproperties preserving the other advantages of polymeric concrete. Thenature of polymeric concrete allows adequate reinforcement throughaddition of fibre reinforced plastic materials or steel, thuscomplementing its mechanical properties, namely flexure strength. Thus,polymeric concrete can be successfully employed as a basis material onits own for large structures. Applied to towers for wind generators andother structural applications, the casting process of polymeric concreteallows plasticity in form modelling in a very cost-effective way, thusallowing adaptation and optimization of the structural design to loadsand logistic restraints, while contributing with its chemical propertiesto reduce maintenance costs.

Towers for large structural applications typically have a base diameterof more than 4 m and reach heights above 50 m, supporting heavy loads.Examples of towers for large structural applications are towers forwindmills, lighthouses or pillars supporting highways in viaducts. Forall of these polymeric concrete can be used as the base material,presenting advantages in lower maintenance and in lower logistics andproduction costs.

The polymeric concrete is prepared by thorough mixing of the binder andfiller materials in adequate ratios, and adding the hardener to promotethe complete polymerization. Large scale production of polymer concreteis performed in proper equipment. Binder and filler have separatehoppers and their mixture is promoted through a screw mechanism.Granulometry and viscosity are controlled to assure the adequate flow ofthe resulting mixture and the final mechanical characteristics of thepolymeric concrete. The finished mixture is then filled into a mould andcompacted through vibration. The moulds are made of steel and/or otheradequate materials. After polymerization the final product is removedfrom the mould.

DESCRIPTION OF THE FIGURES

The annexed drawings exemplify a solution according to the invention:

FIG. 1 shows a tower (1) made of several superimposed horizontalsections, or rings (2), of cast polymeric concrete, with convenient wallthickness.

FIG. 2 shows a cross-section of one section (2), or ring.

FIG. 3 shows the cross-section of another section(2), or ring, beingbuilt of 3 shell segments (3), joined together, using appropriatechemical bonding, like structural glue (4).

FIG. 4 shows how two superimposed rings (5) and (6) are joined togetherusing appropriate chemical bonding, like structural glue (7), betweenconveniently fitted ends, respectively (8) and (9), of the adjoiningsections or rings. The same figure shows an example of a reinforcingelement (10) within the polymeric concrete wall (11), used to adequatelyincrease stiffness of the tower and decrease risk of fatigue failure.The casting process not only allows modelling the internal wall surface,but also allows fitting into the wall any kind of fixtures needed forinstallation of the internal cabling system, stairs, platforms, etc.

FIG. 5 shows an inserted fixture (15) to fix cables, or other means ofhandling the sections or rings, inserted into the polymeric concretewall (16) of a segment or ring.

DETAILED DESCRIPTION OF THE INVENTION

The tower, according to the invention, is built of two or moresuperimposed ring sections in conical or cylindrical form, each ringbeing built of one or more shell segments, these segments are joined bymeans of mechanical and/or chemical bonding and made of pre-castpolymeric concrete.

The segments are moulded after mixture of the binder and fillermaterials, as described above. The binder is a thermosetting resin likepolyester resin, epoxy resin, phenolic resin, vinyl ester resin orothers. Before filling the moulds with polymer concrete, chopped fibresmixed with the thermosetting resin can be sprayed onto the mouldexternal wall, to enhance the tensile resistance of the segment.

The rings have specially designed ends in order to assemble into eachother when building a tower. Between two superimposed rings, as seen inFIG. 4, the one in the bottom has the top end shaped like a step with,from the interior to the exterior, first a bottom horizontal surface(12), second a inclined middle surface (13), and third a top horizontalsurface (14). In turn, the top ring, in each pair of superimposed rings,has a bottom end shaped as an inverted step (12′, 13′, 14′) matching thecorresponding top end surfaces of the bottom ring. This kind of assemblymakes the structure more stable because, as the top ring assembles witha bottom ring, both inclined middle surfaces 13 and 13′ of the rings areuniformly pressed against each other, this way pressing the structuralglue interposed between the two surfaces. Surfaces 12 and 12′, or 14 and14′, are joined by press fit due to the top ring weight or are alsoglued. Furthermore, this configuration provides a conveniently extendedinterface for application of the structural glue at assembly, which canbe subjected to shear stresses, enhancing its effect in providing thenecessary stiffness and mechanical resistance of the tower. To optimizethe joining efficiency and to decrease the tensions applied, the surface13 and 13′ are positioned near the internal surface of the ring.

To help stabilise the tower even more, a post tensioned reinforcingelement (10), such as a steel or a polymeric cable, or a compositeprofile, can be used within the polymeric concrete wall (11) in order toincrease the stiffness of the tower and decrease risk of fatiguefailure. To optimize mechanical efficiency, this reinforcement ispositioned near the external surface of the ring and is stressed whenfixing the top ends at assembly. Furthermore, if this reinforcingelement extends through two or more adjacent rings, it can be used as amechanical joining method between these rings, complementing orsubstituting the structural glue.

These advantages are valid both for cylindrical and conical towersegments.

It should be clear that the described embodiments are simply examples ofexecution of the present invention. Variations and modifications, thatare obvious to a person skilled in the art, can be made within the scopeof the invention and are still protected by the following claims.

1. A tower to support on-shore or off-shore wind generators or otherlarge structures, wherein the said tower is formed by two or moresuperimposed ring sections, each ring comprising one or more shellsegments, which are affixed by means of mechanical and/or chemicalconnection and made of pre-cast polymeric concrete, which is composed ofa thermosetting resin with an aggregate of at least 60% weight dry sandsand/or gravel.
 2. The tower, according to claim 1, comprising a conic,cylindrical or prismatic external shape.
 3. The tower, according toclaim 1, wherein between two superimposed rings, the bottom has the topend shaped like a step comprising, from inside-out, firstly a bottomsurface, then an inclined middle surface, and thirdly a top surface, andin that the top ring, in its turn, comprises in each pair ofsuperimposed rings, has a bottom end shaped as an inverted step thatmatches the counterpart surfaces of the bottom ring.
 4. The tower,according to claim 1, wherein further aggregates up to a maximal contentof 20% weight of the polymeric concrete are used, to enhance chemicaland/or physical properties.
 5. The tower, according to claim 1, whereina reinforcement of composite materials is used within the polymericconcrete wall along one or more superimposed rings.
 6. The tower,according to claim 1, wherein a reinforcement of steel cables is usedwithin the polymeric concrete wall along more than two superimposedrings.
 7. The tower, according to claim 1, wherein reinforcement steelwalls are used in the outer and/or inner surfaces.
 8. The tower,according to claim 1, wherein casting takes place on-site.
 9. The tower,according to claim 2, wherein between two superimposed rings, the bottomhas the top end shaped like a step comprising, from inside-out, firstlya bottom surface, then an inclined middle surface, and thirdly a topsurface, and in that the top ring, in its turn, comprises in each pairof superimposed rings, has a bottom end shaped as an inverted step thatmatches the counterpart surfaces of the bottom ring.
 10. The tower,according to claim 2, wherein further aggregates up to a maximal contentof 20% weight of the polymeric concrete are used, to enhance chemicaland/or physical properties.
 11. The tower, according to claim 3, whereinfurther aggregates up to a maximal content of 20% weight of thepolymeric concrete are used, to enhance chemical and/or physicalproperties.
 12. The tower, according to claim 2, wherein a reinforcementof composite materials is used within the polymeric concrete wall alongone or more superimposed rings.
 13. The tower, according to claim 3,wherein a reinforcement of composite materials is used within thepolymeric concrete wall along one or more superimposed rings.
 14. Thetower, according to claim 3, wherein a reinforcement of compositematerials is used within the polymeric concrete wall along one or moresuperimposed rings.
 15. The tower, according to claim 2, wherein areinforcement of steel cables is used within the polymeric concrete wallalong more than two superimposed rings.
 16. The tower, according toclaim 3, wherein a reinforcement of steel cables is used within thepolymeric concrete wall along more than two superimposed rings.
 17. Thetower, according to claim 4, wherein a reinforcement of steel cables isused within the polymeric concrete wall along more than two superimposedrings.
 18. The tower, according to claim 2, wherein reinforcement steelwalls are used in the outer and/or inner surfaces.
 19. The tower,according to claim 3, wherein reinforcement steel walls are used in theouter and/or inner surfaces.
 20. The tower, according to claim 4,wherein reinforcement steel walls are used in the outer and/or innersurfaces.